1. Field of the Invention
The invention relates to a device for reducing spring tension in the belt strap, particularly in a belt retractor for safety belts in motor vehicles, comprising a winding shaft and a return spring secured in a housing, said device comprising means for producing a force acting against the spring tension, said means being couplable with the winding shaft through a flywheel clutch.
2. Description of the Prior Art
Most modern safety belts are currently equipped with an automatic belt retractor which with the aid of a return spring makes it possible to roll a previously withdrawn belt strap onto a winding shaft equipped with a locking device. In the event of an accident, the winding shaft is blocked or locked by the locking device and, as a result, further withdrawal of the strap is prevented and the strapped-in vehicle occupant is safely retained in his seat.
Automatic belt retractors and other belt-retracting devices are naturally subject to internal friction. This friction results in a force differential between the withdrawing force, on the one hand, and the return force, on the other, said forces acting in the belt strap. Because of this force differential, a greater force is needed to withdraw the strap, i.e., a greater force is acting in the strap, when the strap is retracted and wound up. This force differential is particularly evident or pronounced in three-point belts for which in most cases, a D-ring is provided on top of the door post in the middle of the vehicle, because additional frictional forces must be overcome in this ring. A prerequisite for a properly actuatable belt retractor is that the retracting force of the return spring be sufficient to pull up the strap and the tongue provided thereon, after the belt has been unbuckled, and to take up the slack of the unbuckled strap onto the winding shaft. On the other hand, however, in such an arrangement the return spring exerts undesirable tension particularly on the shoulder of the buckled car occupant which, because of the minimal retracting force available, is frequently annoying. Complaints about this phenomenon are voiced, in particular, after long trips.
Spring tension is always minimized. Because of the weight of the strap tongue and the need for proper, fast retraction after the belt is unbuckled, however, the return spring exerts a force of about 15 newtons at the end of the withdrawing movement.
It should be noted that the annoying return spring tension in the strap is not the retracting or winding force but the withdrawing force. For example, breathing of the vehicle occupant causes a portion of the strap to be periodically withdrawn from the retractor and then again retracted over a long period of time. Even this slight movement can, over a long period, produce an unbearable pressure on the occupant's shoulder.
Several approaches to the prevention of this problem have been considered:
A very long spring could be employed. In this case, the withdrawing force acting in the strap could for a nearly completely withdrawn strap be reduced below the 15 N level of conventional springs. The drawback of this solution is that such a long spring requires a lot of space which in common belt retractors is not available. Moreover, such a spring would be very expensive.
Attempting to minimize friction in the retractor is also very costly because of the expensive construction involved. Moreover, the normally used D-ring would then be very bulky, and since this ring is mounted near the occupant's head, it would also be very dangerous.
Other expensive and, because of excessive size sometimes impossible, solutions include the use of a gear box and the installation of a counterspring; even the use of a brake is disadvantageous. The purpose of all these proposals and solutions is to reduce the force acting in the strap.
In contrast to the other proposals, a device of the aforementioned type is known in which there is only one spring, namely the return spring on the belt retractor, but in which a brack band positioned around the periphery of an external member of a freely turning ball acts as a brake. This brake, which with the aid of a clutch acts only in one direction, however, has the drawback that it acts over practically the entire length of the withdrawn strap. In other words, when the winding shaft turns in the winding direction, the known device restrains the force of the return spring, i.e., it acts against it. This, however, occurs over the entire strap length, i.e., over the entire withdrawing or wind-up length. The resulting drawback is that the strap is not properly wound up. Because of the weight of the loosely hanging strap and of the tongue thereon, the winding action is too slow. This can also endanger the occupant's safety, for example if the occupant after leaning forward to reach the glove compartment suddenly leans back because of a critical traffic situation. In this case, the force of the return spring is not sufficient to take up the slack of the strap fast enough.
Also known are several devices wherein a secondary watch spring is used to counteract the force of the return spring. With this approach, it is possible that within limits of production tolerances the ratio of return spring force to counterspring force may not be exactly maintained, in which case the secondary counterspring could totally neutralize the action of the return spring. In this case, the strap, once withdrawn, is no longer retracted, and the existing belt slack does not protect the occupant in the event of an accident.
It would be desirable, therefore, to improve the device of the aforementioned type for reducing the spring tension in the belt strap by using, as before, only the return spring without a counterspring and yet reducing tension to a desired level, not over the entire length of the withdrawn strap but only over a desired portion of this length.